Regulatory networks controlling mitochondrial quality control in skeletal muscle

Reactive oxygen species promote endurance exercise-induced adaptations in skeletal muscles

The discovery that contracting skeletal muscle generates reactive oxygen species (ROS) was first reported over 40 years ago. The prevailing view in the 1980s was that exercise-induced ROS production promotes oxidation of proteins and lipids resulting in muscle damage. However, a paradigm shift occurred in the 1990s as growing research revealed that ROS are signaling molecules, capable of activating transcriptional activators/coactivators and promoting exercise-induced muscle adaptation. Growing evidence supports the notion that reduction-oxidation (redox) signaling pathways play an important role in the muscle remodeling that occurs in response to endurance exercise training. This review examines the specific role that redox signaling plays in this endurance exercise-induced skeletal muscle adaptation. We begin with a discussion of the primary sites of ROS production in contracting muscle fibers followed by a summary of the antioxidant enzymes involved in the regulation of ROS levels in the cell. We then discuss which redox-sensitive signaling pathways promote endurance exercise-induced muscle adaptation and debate the strength of the evidence supporting the notion that redox signaling plays an essential role in muscle adaptation to endurance exercise training. In hopes of stimulating future research, we highlight several important unanswered questions in this field.

Hypomethylation in promoters of PGC-1α involved in exercise-driven skeletal muscular alterations in old age

Exercise training can significantly improve skeletal muscle mitochondrial function and has been proven to be highly relevant to alterations in skeletal muscle DNA methylation. However, it remains unclear whether late-in-life exercise has an effect on promoter methylation of PGC-1α, a key regulator of mitochondrial biogenesis. Here we employed two distinct exercise modalities, constant medium intensity exercise training (CMIT) and high-intensity interval exercise training (HIIT), to investigate their impacts on PGC-1α expression and methylation regulation in skeletal muscle of aged mice. The results revealed a notable decrease in PGC-1α expression in skeletal muscle of aged mice, accompanied by elevated methylation levels of the PGC-1α promoter, and increased DNA methyltransferase (DNMT) protein expressions. However, both forms of exercise training significantly corrected PGC-1α epigenetic changes, increased PGC-1α expression, and ameliorated skeletal muscle reduction. Furthermore, exercise training led to elevated expression of proteins related to mitochondrial biogenesis and energy metabolism in skeletal muscle, improving mitochondrial structure and function. In conclusion, late-in-life exercise improved skeletal muscle function, morphology, and mitochondria biogenesis, which may be associated with hypomethylation in promoters of PGC-1α and increased content of skeletal muscle PGC-1α. Notably, there was no clear difference between HIIT and CMIT in PGC-1α expression and skeletal muscle function.

Nanodrug Delivery Systems for Myasthenia Gravis: Advances and Perspectives

Myasthenia gravis (MG) is a rare chronic autoimmune disease caused by the production of autoantibodies against the postsynaptic membrane receptors present at the neuromuscular junction. This condition is characterized by fatigue and muscle weakness, including diplopia, ptosis, and systemic impairment. Emerging evidence suggests that in addition to immune dysregulation, the pathogenesis of MG may involve mitochondrial damage and ferroptosis. Mitochondria are the primary site of energy production, and the reactive oxygen species (ROS) generated due to mitochondrial dysfunction can induce ferroptosis. Nanomedicines have been extensively employed to treat various disorders due to their modifiability and good biocompatibility, but their application in MG management has been rather limited. Nevertheless, nanodrug delivery systems that carry immunomodulatory agents, anti-oxidants, or ferroptosis inhibitors could be effective for the treatment of MG. Therefore, this review focuses on various nanoplatforms aimed at attenuating immune dysregulation, restoring mitochondrial function, and inhibiting ferroptosis that could potentially serve as promising agents for targeted MG therapy.

RORα-GABP-TFAM axis alleviates myosteatosis with fatty atrophy through reinforcement of mitochondrial capacity

BACKGROUND

Fat infiltration in muscle, called 'myosteatosis', precedes muscle atrophy, which subsequently results in sarcopenia. Myosteatosis is frequently observed in patients with nonalcoholic fatty liver disease (NAFLD). We have previously reported that retinoic acid receptor-related orphan receptor-α (RORα) regulates mitochondrial dynamics and mitophagy in hepatocytes, resulting in an alleviation of NAFLD. In this study, we aimed to investigate the role of RORα in skeletal muscle and to understand molecular mechanisms by which RORα controls mitochondrial capacity, using an NAFLD-associated myosteatosis mouse model.

METHODS

To establish a myosteatosis model, 7-week-old C57BL/6N mice were fed with high-fat diet (HFD). After 15 weeks of diet feeding, an adeno-associated virus vector encoding RORα (AAV-RORα) was injected to gastrocnemius (GA) muscles, or after 7 weeks of HFD feeding, JC1-40, an RORα agonistic ligand, was administered daily at a dose of 5 mg/kg/day by oral gavage for 5 weeks. Histological, biochemical and molecular analyses in various in vivo and in vitro experiments were performed.

RESULTS

First, the number of oxidative MyHC2a fibres with intensive lipid infiltration increased by 3.8-fold in the red region of the GA of mice with myosteatosis (P < 0.001). RORα was expressed around MyHC2a fibres, and its level increased by 2.7-fold after HFD feeding (P < 0.01). Second, treatment of RORα ligands in C2C12 myoblasts, such as cholesterol sulfate and JC1-40, enhanced the number of oxidative fibres stained for MyHC1 and MyHC2a by two-fold to four-fold (P < 0.01), while it reduced the lipid levels in MyHC2a fibres by 20-50% (P < 0.001) in the presence of palmitic acids. Third, mitochondrial membrane potential (P < 0.01) and total area of mitochondria (P < 0.01) were enhanced by treatment of these ligands. Chromatin immunoprecipitation analysis showed that RORα bound the promoter of GA-binding protein α subunit gene that led to activation of mitochondrial transcription factor A (TFAM) in C2C12 myoblasts (P < 0.05). Finally, intramuscular transduction of AAV-RORα alleviated the HFD-induced myosteatosis with fatty atrophy; lipid contents in MyHC2a fibres decreased by 48% (P < 0.001), whereas the number of MyHC2b fibre increased by 22% (P < 0.001). Also, administration of JC1-40 improved the signs of myosteatosis in that it decreased the level of adipose differentiation-related protein (P < 0.01) but increased mitochondrial proteins such as cytochrome c oxidase 4 and TFAM in GA muscle (P < 0.01).

CONCLUSIONS

RORα plays a versatile role in regulating the quantity of mitochondria and the oxidative capacity, ultimately leading to an improvement in myosteatosis symptoms.

Exercise couples mitochondrial function with skeletal muscle fiber type via ROS-mediated epigenetic modification

Skeletal muscle is a heterogeneous tissue composed of different types of muscle fibers, demonstrating substantial plasticity. Physiological or pathological stimuli can induce transitions in muscle fiber types. However, the precise regulatory mechanisms behind these transitions remains unclear. This paper reviews the classification and characteristics of muscle fibers, along with the classical mechanisms of muscle fiber type transitions. Additionally, the role of exercise-induced muscle fiber type transitions in disease intervention is reviewed. Epigenetic pathways mediate cellular adaptations and thus represent potential targets for regulating muscle fiber type transitions. This paper focuses on the mechanisms by which epigenetic modifications couple mitochondrial function and contraction characteristics. Reactive Oxygen Species (ROS) are critical signaling regulators for the health-promoting effects of exercise. Finally, we discuss the role of exercise-induced ROS in regulating epigenetic modifications and the transition of muscle fiber types.

Exercise, mitochondrial dysfunction and inflammasomes in skeletal muscle

In the broad field of inflammation, skeletal muscle is a tissue that is understudied. Yet it represents about 40% of body mass in non-obese individuals and is therefore of fundamental importance for whole body metabolism and health. This article provides an overview of the unique features of skeletal muscle tissue, as well as its adaptability to exercise. This ability to adapt, particularly with respect to mitochondrial content and function, confers a level of metabolic "protection" against energy consuming events, and adds a measure of quality control that determines the phenotypic response to stress. Thus, we describe the particular role of mitochondria in promoting inflammasome activation in skeletal muscle, contributing to muscle wasting and dysfunction in aging, disuse and metabolic disease. We will then discuss how exercise training can be anti-inflammatory, mitigating the chronic inflammation that is observed in these conditions, potentially through improvements in mitochondrial quality and function.

High-intensity interval training attenuates impairment in regulatory protein machinery of mitochondrial quality control in skeletal muscle of diet-induced obese mice

Mitochondrial quality control processes are essential in governing mitochondrial integrity and function. The purpose of the study was to examine the effects of 10 weeks of high-intensity interval training (HIIT) on the regulatory protein machinery of skeletal muscle mitochondrial quality control and whole-body glucose homeostasis in diet-induced obese mice. Male C57BL/6 mice were assigned to low-fat diet (LFD) or high-fat diet (HFD) group. After 10 weeks, HFD-fed mice were divided into sedentary and HIIT (HFD + HIIT) groups for another 10 weeks (n = 9/group). Graded exercise test, glucose and insulin tolerance tests, mitochondrial respiration, and protein markers of mitochondrial quality control processes were determined. HFD-fed mice exhibited lower ADP-stimulated mitochondrial respiration (p < 0.05). However, 10 weeks of HIIT prevented this impairment (p < 0.05). Importantly, the ratio of Drp1(Ser616) over Drp1(Ser637) phosphorylation, an indicator of mitochondrial fission, was significantly higher in HFD-fed mice (p < 0.05), but such increase was attenuated in HFD-HIIT compared to HFD (-35.7%, p < 0.05). Regarding autophagy, skeletal muscle p62 content was lower in the HFD group than the LFD group (-35.1%, p < 0.05); however, such reduction was disappeared in the HFD + HIIT group. In addition, LC3B II/I ratio was higher in the HFD group than the LFD group (15.5%, p < 0.05) but was ameliorated in the HFD + HIIT group (-29.9%, p < 0.05). Overall, our study demonstrated that 10 weeks of HIIT was effective in improving skeletal muscle mitochondrial respiration and the regulatory protein machinery of mitochondrial quality control in diet-induced obese mice through the alterations of mitochondrial fission protein Drp1 phosphorylations and p62/LC3B-mediated regulatory machinery of autophagy.

Gynostemma Pentaphyllum Increases Exercise Performance and Alters Mitochondrial Respiration and AMPK in Healthy Males

This research aimed to determine the effects of Gynostemma pentaphyllum (G. pentaphyllum) on exercise performance, AMP-activated protein kinase (AMPK), and mitochondrial signaling in human muscle. This randomized double-blind placebo control crossover study provided placebo or 450 mg of G. pentaphyllum dried leaf extract equivalent to 2.25 g of dry leaf per day for four weeks to 16 healthy untrained young males, separated by four weeks wash-out. Following 4-week supplementation with G. pentaphyllum, participants had significantly lower leptin and blood glucose levels and improved time trial performance over 20 km, which corresponded with a higher muscle oxygen flux compared to placebo. Muscle AMPK Thr172 phosphorylation significantly increased after 60 min exercise following G. pentaphyllum supplementation. AMPK Thr172 phosphorylation levels relative to total AMPK increased earlier following exercise with G. pentaphyllum compared to placebo. Total ACC-α was lower following G. pentaphyllum supplementation compared to placebo. While further research is warranted, G. pentaphyllum supplementation improved exercise performance in healthy untrained males, which corresponded with improved mitochondrial respiration, altered AMPK and ACC, and decreased plasma leptin and glucose levels.

High-Intensity Interval Training Attenuates Impairment in Regulatory Protein Machinery of Mitochondrial Quality Control in Skeletal Muscle of Diet-Induced Obese Mice

Mitochondrial quality control processes are essential in governing mitochondrial integrity and function. The purpose of the study was to examine the effects of 10 weeks of HIIT on the regulatory protein machinery of skeletal muscle mitochondrial quality control and whole-body glucose homeostasis in diet-induced obese mice. Male C57BL/6 mice were randomly assigned to a low-fat diet (LFD) or high-fat diet (HFD) group. After 10 weeks, HFD-fed mice were divided into sedentary and HIIT (HFD+HIIT) groups and remained on HFD for another 10 weeks (n=9/group). Graded exercise test, glucose and insulin tolerance tests, mitochondrial respiration and regulatory protein markers of mitochondrial quality control processes were determined by immunoblots. Ten weeks of HIIT enhanced ADP-stimulated mitochondrial respiration in diet-induced obese mice (P < 0.05) but did not improve whole-body insulin sensitivity. Importantly, the ratio of Drp1(Ser 616 ) over Drp1(Ser 637 ) phosphorylation, an indicator of mitochondrial fission, was attenuated in HFD-HIIT compared to HFD (-35.7%, P < 0.05). Regarding autophagy, skeletal muscle p62 content was lower in HFD group than LFD group (-35.1%, P < 0.05), however, such reduction was disappeared in HFD+HIIT group. In addition, LC3B II/I ratio was higher in HFD than LFD group (15.5%, P < 0.05) but was ameliorated in HFD+HIIT group (-29.9%, P < 0.05). Overall, our study demonstrated that 10 weeks of HIIT was effective in improving skeletal muscle mitochondrial respiration and the regulatory protein machinery of mitochondrial quality control in diet-induced obese mice through the alterations of mitochondrial fission protein Drp1 activity and p62/LC3B-mediated regulatory machinery of autophagy.

AMPK is mitochondrial medicine for neuromuscular disorders

Duchenne muscular dystrophy (DMD), myotonic dystrophy type 1 (DM1), and spinal muscular atrophy (SMA) are the most prevalent neuromuscular disorders (NMDs) in children and adults. Central to a healthy neuromuscular system are the processes that govern mitochondrial turnover and dynamics, which are regulated by AMP-activated protein kinase (AMPK). Here, we survey mitochondrial stresses that are common between, as well as unique to, DMD, DM1, and SMA, and which may serve as potential therapeutic targets to mitigate neuromuscular disease. We also highlight recent advances that leverage a mutation-agnostic strategy featuring physiological or pharmacological AMPK activation to enhance mitochondrial health in these conditions, as well as identify outstanding questions and opportunities for future pursuit.

Exercise Improves the Coordination of the Mitochondrial Unfolded Protein Response and Mitophagy in Aging Skeletal Muscle

The mitochondrial unfolded protein response (UPRmt) and mitophagy are two mitochondrial quality control (MQC) systems that work at the molecular and organelle levels, respectively, to maintain mitochondrial homeostasis. Under stress conditions, these two processes are simultaneously activated and compensate for each other when one process is insufficient, indicating mechanistic coordination between the UPRmt and mitophagy that is likely controlled by common upstream signals. This review focuses on the molecular signals regulating this coordination and presents evidence showing that this coordination mechanism is impaired during aging and promoted by exercise. Furthermore, the bidirectional regulation of reactive oxygen species (ROS) and AMPK in modulating this mechanism is discussed. The hierarchical surveillance network of MQC can be targeted by exercise-derived ROS to attenuate aging, which offers a molecular basis for potential therapeutic interventions for sarcopenia.

A cross-talk between sestrins, chronic inflammation and cellular senescence governs the development of age-associated sarcopenia and obesity

The rapid increase in both the lifespan and proportion of older adults is accompanied by the unprecedented rise in age-associated chronic diseases, including sarcopenia and obesity. Aging is also manifested by increased susceptibility to multiple endogenous and exogenous stresses enabling such chronic conditions to develop. Among the main physiological regulators of cellular adaption to various stress stimuli, such as DNA damage, hypoxia, and oxidative stress, are sestrins (Sesns), a family of three evolutionarily conserved proteins, Sesn1, 2, and 3. Age-associated sarcopenia and obesity are characterized by two key processes: (i) accumulation of senescent cells in the skeletal muscle and adipose tissue and (ii) creation of a systemic, chronic, low-grade inflammation (SCLGI). Presumably, failed SCLGI resolution governs the development of these chronic conditions. Noteworthy, Sesns activate senolytics, which are agents that selectively eliminate senescent cells, as well as specialized pro-resolving mediators, which are factors that physiologically provide inflammation resolution. Sesns reveal clear beneficial effects in pre-clinical models of sarcopenia and obesity. Based on these observations, we propose a novel treatment strategy for age-associated sarcopenia and obesity, complementary to the conventional therapeutic modalities: Sesn activation, SCLGI resolution, and senescent cell elimination.

Aerobic exercise training resets the human skeletal muscle methylome 10 years after breast cancer treatment and survival

Cancer survivors suffer impairments in skeletal muscle in terms of reduced mass and function. Interestingly, human skeletal muscle possesses an epigenetic memory of earlier stimuli, such as exercise. Long-term retention of epigenetic changes in skeletal muscle following cancer survival and/or exercise training has not yet been studied. We, therefore, investigated genome-wide DNA methylation (methylome) in skeletal muscle following a 5-month, 3/week aerobic-training intervention in breast cancer survivors 10-14 years after diagnosis and treatment. These results were compared to breast cancer survivors who remained untrained and to age-matched controls with no history of cancer, who undertook the same training intervention. Skeletal muscle biopsies were obtained from 23 females before(pre) and after(post) the 5-month training period. InfiniumEPIC 850K DNA methylation arrays and RT-PCR for gene expression were performed. The breast cancer survivors displayed a significant retention of increased DNA methylation (i.e., hypermethylation) at a larger number of differentially methylated positions (DMPs) compared with healthy age-matched controls pre training. Training in cancer survivors led to an exaggerated number of DMPs with a hypermethylated signature occurring at non-regulatory regions compared with training in healthy age-matched controls. However, the opposite occurred in important gene regulatory regions, where training in cancer survivors elicited a considerable reduction in methylation (i.e., hypomethylation) in 99% of the DMPs located in CpG islands within promoter regions. Importantly, training was able to reverse the hypermethylation identified in cancer survivors back toward a hypomethylated signature that was observed pre training in healthy age-matched controls at 300 (out of 881) of these island/promoter-associated CpGs. Pathway enrichment analysis identified training in cancer survivors evoked a predominantly hypomethylated signature in pathways associated with cell cycle, DNA replication/repair, transcription, translation, mTOR signaling, and the proteosome. Differentially methylated region (DMR) analysis also identified genes: BAG1, BTG2, CHP1, KIFC1, MKL2, MTR, PEX11B, POLD2, S100A6, SNORD104, and SPG7 as hypermethylated in breast cancer survivors, with training reversing these CpG island/promoter-associated DMRs toward a hypomethylated signature. Training also elicited a largely different epigenetic response in healthy individuals than that observed in cancer survivors, with very few overlapping changes. Only one gene, SIRT2, was identified as having altered methylation in cancer survivors at baseline and after training in both the cancer survivors and healthy controls. Overall, human skeletal muscle may retain a hypermethylated signature as long as 10-14 years after breast cancer treatment/survival. Five months of aerobic training reset the skeletal muscle methylome toward signatures identified in healthy age-matched individuals in gene regulatory regions.

Structural functionality of skeletal muscle mitochondria and its correlation with metabolic diseases

The skeletal muscle is one of the largest organs in the mammalian body. Its remarkable ability to swiftly shift its substrate selection allows other organs like the brain to choose their preferred substrate first. Healthy skeletal muscle has a high level of metabolic flexibility, which is reduced in several metabolic diseases, including obesity and Type 2 diabetes (T2D). Skeletal muscle health is highly dependent on optimally functioning mitochondria that exist in a highly integrated network with the sarcoplasmic reticulum and sarcolemma. The three major mitochondrial processes: biogenesis, dynamics, and mitophagy, taken together, determine the quality of the mitochondrial network in the muscle. Since muscle health is primarily dependent on mitochondrial status, the mitochondrial processes are very tightly regulated in the skeletal muscle via transcription factors like peroxisome proliferator-activated receptor-γ coactivator-1α, peroxisome proliferator-activated receptors, estrogen-related receptors, nuclear respiratory factor, and Transcription factor A, mitochondrial. Physiological stimuli that enhance muscle energy expenditure, like cold and exercise, also promote a healthy mitochondrial phenotype and muscle health. In contrast, conditions like metabolic disorders, muscle dystrophies, and aging impair the mitochondrial phenotype, which is associated with poor muscle health. Further, exercise training is known to improve muscle health in aged individuals or during the early stages of metabolic disorders. This might suggest that conditions enhancing mitochondrial health can promote muscle health. Therefore, in this review, we take a critical overview of current knowledge about skeletal muscle mitochondria and the regulation of their quality. Also, we have discussed the molecular derailments that happen during various pathophysiological conditions and whether it is an effect or a cause.

Insight into the Molecular Signature of Skeletal Muscle Characterizing Lifelong Football Players

BACKGROUND

Aging and sedentary behavior are independent risk factors for non-communicable diseases. An active lifestyle and structured physical activity are positively associated with a healthier quality of life in the elderly. Here, we explored the proteomic/metabolomic muscular signature induced by lifelong football training associated with successful aging.

METHODS

The study was performed on nine lifelong football players (67.3 ± 2.8 yrs) and nine aged-matched untrained subjects. We performed a proteomic/metabolomic approach on V. lateralis muscle biopsies; the obtained data were analyzed by means of different bioinformatic tools.

RESULTS

Our results indicated that lifelong football training is able to enhance the muscles' oxidative capacity in the elderly by promoting fatty acids as preferential energetic substrates and hence determining a healthier body composition and metabolic profile; furthermore, we showed that the total polyamine content is higher in lifelong football players' muscle, enforcing the involvement of polyamines in muscle growth and hypertrophy.

CONCLUSIONS

Lifelong football training, as a structured physical activity, significantly influences the expression of the proteins and metabolites involved in oxidative metabolism and muscle hypertrophy associated with successful aging.

ATF5 is a regulator of exercise-induced mitochondrial quality control in skeletal muscle

OBJECTIVES

The Mitochondrial Unfolded Protein Response (UPR^mt) is a compartment-specific mitochondrial quality control (MQC) mechanism that uses the transcription factor ATF5 to induce the expression of protective enzymes to restore mitochondrial function. Acute exercise is a stressor that has the potential to temporarily disrupt organellar protein homeostasis, however, the roles of ATF5 and the UPR^mt in maintaining basal mitochondrial content, function and exercise-induced MQC mechanisms in skeletal muscle are not known.

METHODS

ATF5 KO and WT mice were examined at rest or after a bout of acute endurance exercise. We measured protein content in whole muscle, nuclear, cytosolic and mitochondrial fractions, in addition to mRNA transcript levels in whole muscle. Using isolated mitochondria, we quantified rates of oxygen consumption and ROS emission to observe the effects of the absence of ATF5 on organelle function.

RESULTS

ATF5 KO mice exhibited a larger and less functional muscle mitochondrial pool, most likely a culmination of enhanced biogenesis via increased PGC-1α expression, and attenuated mitophagy. The absence of ATF5 resulted in a reduction in antioxidant proteins and increases in mitochondrial ROS emission, cytosolic cytochrome c, and the expression of mitochondrial chaperones. KO muscle also displayed enhanced exercise-induced stress kinase signaling, but a blunted mitophagic and UPR^mt gene expression response, complemented by significant increases in the basal mRNA abundance and nuclear localization of ATF4. Instead of promoting its nuclear translocation, acute exercise caused the enrichment of ATF5 in mitochondrial fractions. We also identified PGC-1α as an additional regulator of the basal expression of UPR^mt genes.

CONCLUSION

The transcription factor ATF5 retains a critical role in the maintenance of mitochondrial homeostasis and the appropriate response of muscle to acute exercise for the optimization of mitochondrial quality control.

Mitochondrial Dysfunction in Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a devastating neuromuscular disorder caused by recessive mutations in the SMN1 gene, globally affecting ~8-14 newborns per 100,000. The severity of the disease depends on the residual levels of functional survival of motor neuron protein, SMN. SMN is a ubiquitously expressed RNA binding protein involved in a plethora of cellular processes. In this review, we discuss the effects of SMN loss on mitochondrial functions in the neuronal and muscular systems that are the most affected in patients with spinal muscular atrophy. Our aim is to highlight how mitochondrial defects may contribute to disease progression and how restoring mitochondrial functionality may be a promising approach to develop new therapies. We also collected from previous studies a list of transcripts encoding mitochondrial proteins affected in various SMA models. Moreover, we speculate that in adulthood, when motor neurons require only very low SMN levels, the natural deterioration of mitochondria associated with aging may be a crucial triggering factor for adult spinal muscular atrophy, and this requires particular attention for therapeutic strategies.

Endurance Training Increases the Running Performance of Untrained Men without Changing the Mitochondrial Volume Density in the Gastrocnemius Muscle

The activity and quantity of mitochondrial proteins and the mitochondrial volume density (Mito_VD) are higher in trained muscles; however, the underlying mechanisms remain unclear. Our goal was to determine if 20 weeks’ endurance training simultaneously increases running performance, the amount and activity of mitochondrial proteins, and Mito_VD in the gastrocnemius muscle in humans. Eight healthy, untrained young men completed a 20-week moderate-intensity running training program. The training increased the mean speed of a 1500 m run by 14.0% (p = 0.008) and the running speed at 85% of maximal heart rate by 9.6% (p = 0.008). In the gastrocnemius muscle, training significantly increased mitochondrial dynamics markers, i.e., peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) by 23%, mitochondrial transcription factor A (TFAM) by 29%, optic artrophy-1 (OPA1) by 31% and mitochondrial fission factor (MFF) by 44%, and voltage-dependent anion channel 1 (VDAC1) by 30%. Furthermore, training increased the amount and maximal activity of citrate synthase (CS) by 10% and 65%, respectively, and the amount and maximal activity of cytochrome c oxidase (COX) by 57% and 42%, respectively, but had no effect on the total Mito_VD in the gastrocnemius muscle. We concluded that not Mito_VD per se, but mitochondrial COX activity (reflecting oxidative phosphorylation activity), should be regarded as a biomarker of muscle adaptation to endurance training in beginner runners.

Flavanol-Rich Cocoa Supplementation Inhibits Mitochondrial Biogenesis Triggered by Exercise

The potential role of cocoa supplementation in an exercise context remains unclear. We describe the effects of flavanol-rich cocoa supplementation during training on exercise performance and mitochondrial biogenesis. Forty-two male endurance athletes at the beginning of the training season received either 5 g of cocoa (425 mg of flavanols) or maltodextrin (control) daily for 10 weeks. Two different doses of cocoa (equivalent to 5 g and 15 g per day of cocoa for a 70 kg person) were tested in a mouse exercise training study. In the athletes, while both groups had improved exercise performance, the maximal aerobic speed increased only in the control group. A mitochondrial DNA analysis revealed that the control group responded to training by increasing the mitochondrial load whereas the cocoa group showed no increase. Oxidative stress was lower in the cocoa group than in the control group, together with lower interleukin-6 levels. In the muscle of mice receiving cocoa, we corroborated an inhibition of mitochondrial biogenesis, which might be mediated by the decrease in the expression of nuclear factor erythroid-2-related factor 2. Our study shows that supplementation with flavanol-rich cocoa during the training period inhibits mitochondrial biogenesis adaptation through the inhibition of reactive oxygen species generation without impacting exercise performance.